1. Field of the Invention
The present invention relates to a copper alloy type sintered friction material, and more specifically, to an improvement of corrosion resistance and various mechanical properties such as wear resistance, seizure resistance, coefficient of friction, strength, toughness, hardness and so on of a sintered friction material preferably used for friction sliding parts such as a clutch or a brake or for mechanical sliding parts such as a synchronizing ring, of a vehicle (including a motorcycle).
2. Description of the Related Art
In a vehicle driving system, of which the performance has constantly been improved, a high torque is applied. Therefore, materials of parts used therein have been improved, designs have been studied and, in addition, a lubricant oil, which is one factor of the environment of use, has been improved. For example, an extreme pressure additive to oil based on phosphorus (P) or sulfur (S) has an effect of reducing damage due to wear of the friction sliding parts caused under severe sliding conditions. However, it is well known that products such as hydrogen sulfide (H.sub.2 S) generated by decomposition of the extreme pressure additive at a high temperature react with non-ferrous metal, causing corrosion of the material.
Concerning the problem of corrosion, brass (copper-zinc alloy) has superior resistance to corrosion due to sulfur. At present, brass is practically used for mechanical parts that are to be employed in such an environment susceptible to corrosion due to sulfur. However, wear resistance and seizure resistance of brass alloy are not superior, and the material has not been designed to have a high coefficient of friction in oil, and hence application of brass alloy as a friction sliding member has been is difficult.
With respect to this problem, Japanese Patent Publication No. 4-80105 discloses that a cast material provided by adding an intermetallic compound of nickel silicide and a metal component such as iron, aluminum, and manganese or the like to a copper-zinc-nickel type alloy realizes a coefficient of friction within the range of from about 0.1 to 0.13 in oil, and that the cast material can be utilized for a synchronizing ring.
While higher grade and higher performance of vehicles have been demanded, it has been found that the driver's shifting feel when he or she shifts the transmission gear can be much improved by improving the coefficient of friction of the synchronizing ring, which is a part of the transmission system. However, a conventional sliding member formed of a copper type alloy including the one disclosed in the aforementioned Japanese Patent Publication No. 4-80105 has the coefficient of friction in oil in the range about 0.1 to 0.15, and hence it cannot exhibit sufficient frictional effect to improve the shifting feel of the driver.
Now, a copper alloy type sintered friction material has been conventionally known as a friction material having relatively high coefficient of friction and, for example, has been disclosed in Japanese Patent Laying-Open Nos. 58-79073, 58-151444, 60-116751, 61-67737, 63-109131 and so on. In the sintered friction materials, in order to improve the coefficient of friction by causing friction resistance at the time of sliding, hard particles or a friction adjusting agent are simply mixed with the copper alloy powder, and the mixed powder is molded and sintered. Therefore, microscopically, even after sintering, the hard particles or the friction adjusting agent do not form a reaction layer with the base copper alloy, but exist with spaces left between the grain boundaries of the original copper alloy powder (especially at triple point of grain boundaries).
As a result, in the conventional copper alloy type sintered friction material, the hard particles or the friction adjusting agent become loose and fall out from the grain boundaries of the sintered material (especially from the triple point of grain boundaries) at the time of friction sliding, and thus form an abraded powder, which powder attacks the counterpart or the sintered friction material itself, causing seizure or damage due to wear. Further, in the conventional copper alloy type sintered friction material, the hard particles or the friction adjusting agent do not have an appropriate grain diameter, and are not dispersed uniformly. Therefore, it is difficult to realize a stable coefficient of friction of at least 0.2 under wet type sliding and at least 0.4 under dry type sliding, which values are required of the high performance friction material.
As described above, it has been desired to provide a copper alloy type sintered friction material having superior resistance to corrosion caused by sulfur, superior wear resistance and superior seizure resistance and stable, relatively high coefficient of friction in oil.